(1) Field of the Invention
This invention relates to novel imidazole compounds, a process for synthesizing these compounds, and a method of curing epoxy resins using these compounds.
The compounds obtained by this invention are novel, and useful as a curing agent or a curing accelerator for epoxy resins.
(2) Description of the Prior Art
Nowadays, various imidazole compounds are used as curing agents or curing accelerators in the epoxy resin industry. Among them, 2-ethyl-4-methylimidazole (2E4MZ for short) is by far most frequently used throughout the world.
2E4MZ is intrinsically a normally crystalline substance having a melting point of about 45.degree. C. After it is purified by distillation under reduced pressure, it is liquid at ordinary temperature for a considerably long period of time owing to an overcooling phenomenon. Such liquid 2E4MZ has excellent compatibility with liquid epoxy resins, and this is presumably one reason why it finds worldwise extensive use. However, since it is crystalline at ordinary temperatures, breaking of the overcooling phenomenon by some cause results in its solidification. At least the following empirical knowledge exists with regard to the cause of breaking overcooling.
It is known that a crystal nucleus sometimes forms in liquid 2E4MZ when a certain physical stimulation (for example, vigorous vibration or rubbing of the inside surface of the container by a pointed solid object) is given to it or a certain minute foreign material is included in it, and that once the crystal nucleus forms, crystals grow from it and soon the entire 2E4MZ solidifies. It is also known that liquid 2E4MZ entirely solidifies almost surely when the crystals of 2E4MZ are finely pulverized and added to liquid 2E4MZ (so-called nucleation).
When liquid 2E4MZ solidifies, its compatibility with a liquid epoxy resin is naturally inferior to that of liquid 2E4MZ. The solidified 2E4MZ becomes liquid when melted by heating. This, however, is troublesome, and the user dislikes the solidification.
2E4MZ is now commercially available in various purities. The ease or difficulty of its solidification depends upon its purity. Generally, solidification is easier for higher purities, and more difficult for lower purities. The present applicants are manufacturing and selling 2E4MZ, which are purified by distillation under reduced pressure and supply to the industry relatively high-purity grades having an average purity of 97% and a Gardner number of 2 to 3. They are however liable to solidify, and particularly in the wintertime, there is a tendency to accelerated solidification. 2E4MZ manufactured by a certain company in West Germany has an average purity of as high as 97%, and therefore readily solidifies.
In contrast, 2E4MZ manufactured by a certain company in U.S.A. has a purity of as low as 82% and contains considerable amounts of impurities. Its color hue is expressed by a Gardner number of at least 18 (scale out). Hence, this 2E4MZ is difficult of solidification, but because of its low purity, it often fails to attract the user's interest.
No product which has a high purity and does not solidify is found among 2E4MZ products now commercially available.
Users who dislike solidification must equip themselves with a device for keeping 2E4MZ warm to maintain it liquid. Although there is a strong demand in the industry for the advent of 2E4MZ which does not solidify, the advent of such 2E4MZ is impossible judging from its inherent property. In short, the industry has sought a method of solving a problem which is impossible of solution.
Extensive investigations of the present inventors have led to the discovery that a certain class of 1-aminoethylimidazole compounds are liquid, and that these compounds have properties equivalent to 2E4MZ as curing agents or curing accelerators for epoxy resins. The problems associated with 2E4MZ are solved by the provision of 1-aminoethyl-imidazole compounds of the invention.
Means of solving these problems will now be described.
First, a method of synthesizing a 1-acylaminoethyl-imidazoline compound which becomes a starting material for the 1-aminoethyl-imidazole compound will be described.
This synthesis is carried out by any one of the following two methods using diethylenetriamine (DETA for short). ##STR2##
In the above reaction, RCN and RCOOH are used each in an amount of 1 to 1.2 moles per mole of DETA. Desulfurization of the 1-aminoethyl-imidazoline compound is carried out by heating the reaction mixture with an alkali hydroxide, zinc dust or iron dust, filtering the heated mixture, and distilling the filtrate under reduced pressure or recrystallizing it. The reaction of the desulfurized 1-aminoethyl-imidazoline compound with RCOOH is carried out while evaporating the water formed. When the evaporation ceases, the reaction mixture is subjected to dehydrogenation. ##STR3##
In method (2), 2 to 2.4 moles of RCOOH or 1 to 1.2 moles of RCO O COR (acid anhydride) is used per mole of DETA.
The reaction is carried out while evaporating the water formed. When the evaporation ceases, the resulting mixture is subjected to dehydrogenation.
The present inventors found that the 1-aminoethyl-imidazoline compound, a precursor of the 1-acylaminoethyl-imidazoline compound in method (1) does not undergo dehydrogenation in the presence of the catalyst used in this invention. Extensive investigations have led to the discovery that the dehydrogenation reaction proceeds if it is acylated to a 1-acylaminoethyl-imidazoline compound. This discovery has led to the present invention.
Then, 1-acylaminoethyl-imidazoline compound is heated together with a metal catalyst (Group VIII of the periodic table) at a temperature of 150.degree. to 270.degree. C. whereby it is dehydrogenated to a 1-acylaminoethyl-imidazole compound. This is schematically shown as follows. ##STR4##
Ni, Co, Pd and Pt can be used as the metal catalyst. In view of economy, Ni is most preferably used. Particularly, the use of commercially available stabilized nickel (a product of Nikki Chemical Co., Ltd.) is preferred. The stabilized nickel is used in an amount of 5 to 10% by weight based on the imidazoline compound.
The dehydrogenation usually ends within 1 hour. The dehydrogenation reaction mixture is filtered, and the metal catalyst is separated by filtration and recovered. The filtrate is subjected to distillation under reduced pressure or to recrystallization to obtain the 1-acylaminoethyl-imidazole compound. The 1-acylaminoethyl-imidazole compound is important as a precursor of a 1-aminoethyl-imidazole compound.
The resulting 1-acylaminoethyl-imidazole compound is a novel substance. Hydrolysis and subsequent purification in a customary manner give the final desired 1-aminoethyl-imidazole compound.
Hydrolysis is carried out using an alkali hydroxide or sulfuric acid. As a solvent for this reaction, there may be used water, hydrous ethylene glycol, hydrous diethylene glycol or hydrous methyl Cellosolve R (monomethoxyethylene glycol). Most of the 1-aminoethyl-imidazole compounds obtained by the above procedure are liquid and novel compounds. They are useful as curing agents or curing accelerators for epoxy resins.
The properties of the compounds of this invention are shown below.